Integration of electrical resistivity imaging and ground penetrating radar to investigate solution features in the Biscayne Aquifer

•Electrical and GPR anomalies seen below 4 m depth within the saturated zone.•High electrical anisotropy (λ = 1.24-1.36) with minimum resistivity trending E-W.•Low resistivity zones (ρ ⩽ 40 Ω-m) observed on resistivity profiles.•Numerous diffractions and low EM wave velocities (0.07-0.08 m ns−1) on GPR profiles.•Geophysical anomalies consistent with high porosity zones in nearby borehole images.

SummaryThe limestone composing the Biscayne Aquifer in southeast Florida is characterized by cavities and solution features that are difficult to detect and quantify accurately because of their heterogeneous spatial distribution. Such heterogeneities have been shown by previous studies to exert a strong influence in the direction of groundwater flow. In this study we use an integrated array of geophysical methods to detect the lateral extent and distribution of solution features as indicative of anisotropy in the Biscayne Aquifer. Geophysical methods included azimuthal resistivity measurements, electrical resistivity imaging (ERI) and ground penetrating radar (GPR) and were constrained with direct borehole information from nearby wells. The geophysical measurements suggest the presence of a zone of low electrical resistivity (from ERI) and low electromagnetic wave velocity (from GPR) below the water table at depths of 4-9 m that corresponds to the depth of solution conduits seen in digital borehole images. Azimuthal electrical measurements at the site reported coefficients of electrical anisotropy as high as 1.36 suggesting the presence of an area of high porosity (most likely comprising different types of porosity) oriented in the E-W direction. This study shows how integrated geophysical methods can help detect the presence of areas of enhanced porosity which may influence the direction of groundwater flow in a complex anisotropic and heterogeneous karst system like the Biscayne Aquifer.